Field of the Invention.
The present invention relates to a color image reading apparatus or a color image forming apparatus, and more particularly to a color image reading apparatus or a color image forming apparatus capable of performing color adjustment.
A conventional digital color copying machine has an electrical processing system as shown in FIG. 2. An original 1 is read with a solid state image pickup device 6. The R, G and B read-out signals undergo logarithmic conversion by a logarithmic converter 15 to obtain Y (yellow), M (magenta) and C (cyan) print output signals after being subjected to a masking process by a primary converter 16. The obtained signals are used for controlling the quantity of color toner or ink to produce a copy of the original. It is difficult for a printer of the color copying machine to obtain a high fidelity reproduction signal from a color signal (stimulus value) of the original. The reason for this is as follows:
The primary objective of copying is to have a copy 21 of a same color as that of the original 1, namely to satisfy the condition of color matching of X.ident.X', Y.ident.Y' and Z.ident.Z' wherein X', Y'and Z' represent tristimulus values of a copy and X, Y and Z represent tristimulus values of an original. To this end, it is essential for the characteristic of a reading system to follow the principle of a chromaticity meter.
Assuming that the characteristic of a printer is defined by a conversion function f from input signals (Y, M and C) into copied tristimulus values (X' Y' and Z'), a total conversion h x g by a color processing system 15 and 16 having a conversion factor h and a reading system having a conversion factor g should have an inverse conversion function f.sup.-1. However, the function f.sup.-1 is non-linear and complicated so that such conversion is hard to be implemented with electronic circuits. In view of this, a simplified and approximate circuit composed of a logarithmic converter 15 and a primary converter 16 is used in practice.
Therefore, it has been difficult to have a copy of exactly the same color as that of an original 1.
Specifically, in case that a change in time occurs of the brightness of an original illuminating lamp, the spectral sensitivity of a photoelectric color conversion element array, or the gain of an amplifier used for digitalizing an image signal, it is impossible to obtain a high fidelity reproduction signal of a color original if color correction is conducted in accordance with predetermined color conversion coefficients.
As a color image forming apparatus, there are known a heat sensitive color transfer printer, an ink-jet color printer, a laser beam color printer, a display unit such as a CRT and the like. Since each apparatus has a different color reproduction characteristic, it become necessary to perform color readjustment when one apparatus is replaced by another apparatus. Namely, since the color reproduction characteristic of an output unit connected to a color image reading apparatus differs from each other and hence has a different optimum setting value for color adjustment, it has been necessary to perform cumbersome color adjustment.
For such a color change, it is difficult for a layman to conduct a color balance adjustment, conventionally, in order to check the characteristic of a printer, a gradation pattern with different color densities generated by a pattern generator has been visually checked. In this case, however, a user other than a color adjustment expert cannot properly perform color adjustment when the color of an original is reproduced quite differently.
As described above, conventional apparatuses pose a problem that a high fidelity color reproduction cannot be obtained due to a different characteristic of a unit in the apparatus and a change in time of the characteristic.